Two events can look the same in a calendar and still be different records. Before removing anything, export a backup and distinguish exact copies from revisions and repeated occurrences.
What the event ID means
UID is the persistent identity of a calendar object. A recurring family uses the same UID; an edited occurrence also has a RECURRENCE-ID to identify the occurrence it changes. Treating every shared UID as a duplicate would delete valid exceptions.
Changed versions need a decision
Two records with the same UID and recurrence identity can contain different summaries, descriptions, dates or revision metadata. A cleanup tool should not choose solely by the visible title. CalPorter keeps both versions and reports the conflict. It does not claim to know which version your calendar account considers current.
What our exact-copy cleanup removes
The tool compares parsed event properties and nested components, including reminders. It ignores the ordering of properties for comparison, but requires their values to match. Identical copies are removed; changed records stay. Two otherwise identical events with different UIDs stay because they might be separate events.
What cleanup cannot do
It does not connect to your account or delete live entries. Reimporting a cleaned file into the original calendar can still leave existing duplicates there. Test in a new calendar first, inspect the result, and decide how to replace or reconcile the old data using your calendar application.
Sources: RFC 5545 UID and RECURRENCE-ID semantics and Microsoft’s calendar import guidance.